1. Field of the Invention
The present invention relates to two-stage binders, a mat making and preforming process, and to apparatus for carrying out the process, for curing binders on non-woven reinforcing materials, woven reinforcing materials and combinations thereof during their manufacture, and is more particularly concerned with utilizing directed energy which is focused for reaction on a two-stage binder without involving the reinforcing materials to a significant degree.
2. Description of the Prior Art
Heretofore, it has been common in the an of manufacturing glass fiber mats, woven materials and other reinforcing materials to apply a binder to both assist in holding the reinforcing material together and promoting a better bond between a matrix resin and the reinforcing material during a subsequent RIM (reaction injection molding), RTM (resin transfer molding) or SPIM (structural reaction injection molding) molding process. These binders are usually dry, powder resins, but can be emulsions or liquids. The fiber materials are produced in a conventional manner for the type of construction desired. Normally, the binders are applied to the reinforcements and then subjected to heating, to melt, or dry- before-melt, and sometimes to cure the binders. This process uses significant quantities of energy as the entire mass of reinforcing material needs to be heated to the required melting and/or drying and/or reaction temperatures. The binder can be either unsaturated, cured or staged, depending on application requirements. In this connection, one may refer to U.S. Pat. No. 4,054,713 of Sakaguchi, et at, fully incorporated herein by this reference, particularly to column 4, beginning at line 27.
Also of interest is U.S. Pat. No. 4,012,553, issued Mar. 15, 1977, in which a partial cure by ultraviolet radiation is disclosed.
In conventional systems, many disadvantages are evident such as the large amount of energy required for involving the entire mass of reinforcing material, the size of the ovens required, the time required for heating and cooling and the production of volatiles that must be collected to prevent contamination of the environment.
It was therefore an object of the invention disclosed in my application for U.S. Pat. No. 685,896, filed Apr. 16, 1991, to provide an improved mat forming process which is fast, efficient in its energy requirements, and safe from an environmental standpoint.
That object was achieved by providing a process and an apparatus for carrying out the process in which a layer of reinforcing fibers is formed on a traveling web, such as a conveyor belt, binder is applied to thinly coat the layer without filling the interstices between the fibers, for example by spraying or calendaring, the binder-coated layer is compressed to a desired density/thickness ratio, and the binder is cured during compression. The resulting mat is then taken from the web and rolled up.
The binder in that application was selected as an electromagnetically-curable binder, either an ultraviolet energy-curable binder or a microwave energy-curable binder. Accordingly, during curing, the corresponding ultraviolet or microwave energy is applied while the layer is compressed to the desired density/thickness ratio. Here, ultraviolet is considered to be from deep .mu.V (.apprxeq.200 nm) up to visible light (.apprxeq.450 nm). Combinations of wavelengths may be used to advantage in practicing the invention.
As is readily apparent, the process and apparatus of the above-identified invention differs significantly from that heretofore known in the art in that directed energy is used to cure the special binders developed for the process, the binders being sensitive to the directed energy used, preferably microwave or ultraviolet. The binders can be cured on the fiber with directed energy without involving the mass of fibers in the energy of reaction. The binders are typically oligomers, unsaturated polyesters, epoxides, polyacrylics, polyurethanes, and the like. When cured, the binders have unsaturated sites available for subsequent bonding with matrix resins in subsequent molding processes such as RTM and SRIM processes. Using this process, the reinforcing material is primarily passive to the activation energy. The reaction takes place without involving the reinforcing materials since the activation energy is targeted only at the binder. The binders perform the same functions as in the conventional manufacturing systems, but can be cured much more rapidly, with significantly less energy and without producing significant volatiles that have to be collected to prevent contamination of the environment. Because there are basically no volatiles, there is little or no need to collect the volatiles driven off by heat, there is little or no need for make-up air and little or no need for make-up heat, thereby saving energy and thereby substantially avoiding contamination of the environment.
As mentioned, the directed-energy source can be either ultraviolet light energy or microwave energy as in my co-pending applications, Ser. No. 446,859 filed Dec. 6, 1989, Ser. No. 552,253 filed Jul. 12, 1990, and Ser. No. 609,420, filed Nov. 5, 1990, all relating to the production of three-dimensional preforms using ultraviolet or microwave energy-curable binders and corresponding ultraviolet or microwave energy sources for curing. Contrary to the aforementioned applications, which disclose stepped forming processes, the process of forming a mat in Ser. No. 685,896, filed Apr. 16, 1991, is continuous.